1. Field of the Invention
The present invention relates to a method of measuring jitter/wander in a video apparatus for processing an input video signal in synchronization with an external reference signal.
2. Description of the Related Art
In the field of video production or the like, it has been typically practiced to build a video synchronization system in which a plurality of video apparatuses are synchronized with a reference signal. FIG. 1 is a schematic block diagram exemplarily showing such video synchronization system. An external reference signal ref is supplied to a plurality of video cameras 50, a switcher 60 and a plurality of VTRs 70 from a reference signal generator 80.
The respective video cameras 50 use a clock generated from the external reference signal ref at a PLL 51 as an internal reference clock of the apparatus. Parallel digital video data generated in an image-capturing system and video signal processing system (not illustrated) is converted into a serial digital signal (hereinafter referred to as an SDI signal) of SDI (Serial Digital Interface) format using the internal reference clock in an SDI output circuit 52. The converted SDI signal is output to the switcher 60.
The switcher 60 is a video apparatus for switching the output of video signals input from the plurality of video apparatuses to the respective video apparatuses. The switcher 60 uses the clock generated from the external reference signal ref at a PLL 61 as the internal reference clock of the apparatus. The switcher 60 decodes the SDI signal input from the respective video cameras 50 into the original parallel digital video data using the internal reference clock at respective SDI input circuits 62. The decoded parallel digital video data is switched at a signal switching unit 63 and afterward converted again into the SDI signal at an SDI output circuit 64, and the converted SDI signal is then output to the respective VTRs 70.
The VTRs 70 decode the SDI signal input from the switcher 60 into the original parallel digital video data at SDI input circuits 72 using the clock generated from the external reference signal ref at a PLL 71 as the internal reference clock of the apparatus. Then, the decoded parallel digital video data is recorded on a video tape in a recording video signal processing system (not illustrated).
In the case where a video signal (an SDI signal in FIG. 1) sent to and received from the video apparatuses carries a large amount of jitter/wander, video data may be affected, because such jitter/wander is not sufficiently absorbed in the video apparatuses (the switcher 60 and VTRs 70 in FIG. 1) inputting the video signal in such video synchronization system.
Accordingly, system debugging using a dedicated instrument for measuring the jitter/wander has been carried out in the past. However, there are cases having a difficulty in identifying the cause of the jitter/wander due to reasons listed in the following [1] through [4]. As a result, there occurs a delay in countermeasure, which is disadvantageous for the user.
[1] Since the respective video apparatuses operate in synchronization with the internal reference clock of the apparatus generated from the external reference signal ref at respective PLLs as shown in FIG. 1, jitter/wander may be caused by variations of reference clock in each device.
[2] Although there is the rated value in SMPTE with respect to an allowable amount of jitter, there is no rated value with respect to wander and therefore a tolerance for the wander in a video apparatus may be unclear.
[3] It is difficult to measure jitter/wander occurring suddenly or less frequently (for example, the jitter/wander caused by a temporary power-supply noise), because the jitter/wander may not occur while the measurement is carried out using the measuring instrument.
[4] A phenomenon similar to large jitter/wander may occur when the reference signal is not supplied (reference failure) due to a cause such as a contact failure of cables connecting the reference signal generator and the video apparatus.
Here, such an optical transmission communication apparatus, for example, as described below has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2003-32212). In the optical transmission communication apparatus, a clock generated in a DMUX portion for demultiplexing a transmission signal input from the outside is used as a writing clock for writing the demultiplexed data in a buffer memory. The clock generated by smoothing out the clock generated in the DMUX portion is used as the reading clock for reading the data from the buffer memory. Further, only the case having a phase close to or distant from the timing writing to and reading from the buffer memory is counted up. Then, as a result of observation for a long time, whether the wander has occurred is determined based on whether a ratio of plus (+) side count value counted on the phase having the read timing close to the write timing to minus (−) side count value counted on the phase having the read timing distant from the write timing is equal.
However, in the case where the jitter has occurred, the ratio of the plus (+) side count value to the minus (−) side count value may be substantially equal in a comparison between the plus (+) side count value and the minus (−) side count value as described in the above Patent Document. Therefore, it may be difficult to determine whether the jitter has occurred.
Further, the optical transmission communication apparatus described in the above Japanese Unexamined Patent Application Publication No. 2003-32212 may be not included in a video synchronization system and not operated in synchronization with the external reference signal.